My son has had a persistent diaper rash. I have tried an anti-fungal cream, Vaseline, Desitin, and also a cortisone cream. My son has eczema and has a prescription strength cream for it. I have used this also. He seems to have some eczema on his wrists that I can't get rid of either. He has also had two bouts of thrush. It has been about a month since that has cleared up.
First, one of the places that eczema doesn't appear is the diaper area. Eczema has a preference for dry parts of the body and the diaper area is one of the wettest. A fungus called candida, however, loves nothing more than wet, dark places and shows up as a bright red rash with separate red spots on the periphery of the rash.
The other common rash is diaper dermatitis, which comes as the skin in this area gets irritated from the moisture and the urine and stool. An anti-fungal ointment like Nystatin used three times a day for a good week should clear up a candida infection. Diaper dermatitis usually responds to frequent diaper changes, air drying (although I know this can get messy), and a barrier ointment like Desitin or A&D ointment.
If you've already tried these remedies with no relief, it's time to let your pediatrician take a look.
As a reminder, the eczema prescription medicine you used could have a fungal infection much worse. Use prescription items only for which they were prescribed.